# 5.6 An Example: Streams of Primes¶

We conclude this chapter with an example of the creation and manipulation of infinite streams of prime integers. This might be useful for experiments with numbers or other applications where you are using sequences of primes over and over again. As for all streams, the stream of primes is only computed as far out as you need. Once computed, however, all the primes up to that point are saved for future reference.

Two useful operations provided by the FriCAS library are prime?prime?IntegerPrimesPackage and nextPrimenextPrimeIntegerPrimesPackage. A straight-forward way to create a stream of prime numbers is to start with the stream of positive integers [2,..] and filter out those that are prime.

Create a stream of primes.

primes : Stream Integer := [i for i in 2.. | prime? i]


 [2,3,5,7,11,13,17,19,23,29,…]

Type: Stream Integer

A more elegant way, however, is to use the generategenerateStream operation from Stream. Given an initial value a and a function f, generategenerateStream constructs the stream [a,f(a),f(f(a)),...]. This function gives you the quickest method of getting the stream of primes.

This is how you use generategenerateStream to generate an infinite stream of primes.

primes := generate(nextPrime,2)


 [2,3,5,7,11,13,17,19,23,29,…]

Type: Stream Integer

Once the stream is generated, you might only be interested in primes starting at a particular value.

smallPrimes := [p for p in primes | p > 1000]


 [1009,1013,1019,1021,1031,1033,1039,1049,1051,1061,…]

Type: Stream Integer

Here are the first 11 primes greater than 1000.

[p for p in smallPrimes for i in 1..11]


 [1009,1013,1019,1021,1031,1033,1039,1049,1051,1061,…]

Type: Stream Integer

Here is a stream of primes between 1000 and 1200.

[p for p in smallPrimes while p < 1200]


 [1009,1013,1019,1021,1031,1033,1039,1049,1051,1061,…]

Type: Stream Integer

To get these expanded into a finite stream, you call completecompleteStream on the stream.

complete %


 [1009,1013,1019,1021,1031,1033,1039,1049,1051,1061,…]

Type: Stream Integer

Twin primes are consecutive odd number pairs which are prime. Here is the stream of twin primes.

twinPrimes := [ [p,p+2] for p in primes | prime?(p + 2)]


 [[3,5],[5,7],[11,13],[17,19],[29,31],[41,43],[59,61],[71,73],..[101,103],[107,109],…]

Type: Stream List Integer

Since we already have the primes computed we can avoid the call to prime?prime?IntegerPrimesPackage by using a double iteration. This time we’ll just generate a stream of the first of the twin primes.

firstOfTwins:= [p for p in primes for q in rest primes | q=p+2]


 [3,5,11,17,29,41,59,71,101,107,…]

Type: Stream Integer

Let’s try to compute the infinite stream of triplet primes, the set of primes p such that [p,p+2,p+4] are primes. For example, [3,5,7] is a triple prime. We could do this by a triple for iteration. A more economical way is to use firstOfTwins. This time however, put a semicolon at the end of the line.

Create the stream of firstTriplets. Put a semicolon at the end so that no elements are computed.

firstTriplets := [p for p in firstOfTwins for q in rest firstOfTwins |


q = p+2];

Type: Stream Integer

What happened? As you know, by default FriCAS displays the first ten elements of a stream when you first display it. And, therefore, it needs to compute them! If you want no elements computed, just terminate the expression by a semicolon (;). The semi-colon prevents the display of the result of evaluating the expression. Since no stream elements are needed for display (or anything else, so far), none are computed.

Compute the first triplet prime.

firstTriplets.1


 3

Type: PositiveInteger

If you want to compute another, just ask for it. But wait a second! Given three consecutive odd integers, one of them must be divisible by 3. Thus there is only one triplet prime. But suppose that you did not know this and wanted to know what was the tenth triplet prime.

firstTriples.10


To compute the tenth triplet prime, FriCAS first must compute the second, the third, and so on. But since there isn’t even a second triplet prime, FriCAS will compute forever. Nonetheless, this effort can produce a useful result. After waiting a bit, hit Ctrl-c. The system responds as follows.

>> System error:
Console interrupt.
You are being returned to the top level of
the interpreter.


If you want to know how many primes have been computed, type:

numberOfComputedEntries primes


and, for this discussion, let’s say that the result is 2045. How big is the 2045-th prime?

primes.2045


 17837

Type: PositiveInteger

What you have learned is that there are no triplet primes between 5 and 17837. Although this result is well known (some might even say trivial), there are many experiments you could make where the result is not known. What you see here is a paradigm for testing of hypotheses. Here our hypothesis could have been: there is more than one triplet prime. We have tested this hypothesis for 17837 cases. With streams, you can let your machine run, interrupt it to see how far it has progressed, then start it up and let it continue from where it left off.